Imprint lithography is gaining interest as a viable alternative to more traditional (mask-based) optical lithography techniques as imprint lithography promises to be able to provide smaller feature sizes in a pattern to be transferred onto a substrate such as the substrate of a semiconductor device. In imprint lithography techniques such as substrate conformal imprinting lithography (SCIL), a flexible stamp including a feature pattern on its surface is brought into contact with such a substrate, which substrate typically carries a resist material, which resist material is imprinted by the feature pattern. The resist material is subsequently developed, e.g. cured, after which the feature pattern is released from the resist material to leave a patterned resist layer on the substrate.
An important aspect in this type of technology has been to achieve nanometer accuracy overlay alignment on wafer scale areas, to allow a so-called “nanoimprint” technology to be enabled for multilayer device manufacture. This would allow the technique to be used for multi-layer device fabrication.
Previous methods have used small area rigid or hard stamps which then need to be aligned and imprinted many times to populate a whole wafer. Alternatively, a single rigid wafer scale stamp is used to avoid distortions in the stamp, but this requires high pressures and forces to be applied to the stamp and substrate in order to make conformal contact. Furthermore, the stamp and the substrate need to be aligned with the final precision before contact is made, avoiding additional shifts and deformations during contacting, because once the two plates are in contact any position error cannot then be corrected.
Previously, soft-stamp imprint lithography methods could not achieve accurate overlay alignment below tens of microns, as the soft stamp would introduce strong distortions. However, it has been shown that with SCIL and with a proper stamp design and stamp placement tooling, non-distorted imprints and nanometer accuracy overlay can be achieved.
The SCIL process makes use of a plate having an array of channels to which a vacuum (negative relative pressure) or a positive pressure can be applied. The pressure controls the suction of a stamp against the plate, or else the pushing of the stamp against the target substrate. In the commercial SCIL tool the alignment procedure involves placing the stamp in a flat configuration on the plate, with all channels applying a vacuum. The substrate is aligned with respect to the stamp. A shift-offset correction method is then applied. This involves bringing a bulge of the stamp into contact with the substrate, and measuring the alignment when there is contact.
The bulge is released so that contact is removed, and position adjustments can be made, before the bulge is again brought into contact with the substrate.
This correction process takes time. First, the shift-offset has to be determined. The eventual overlay error is caused both by the shift-offset and the tool alignment error. The shift-offset is caused by the stamp being initially bulged in order to bridge the gap to the substrate. This bulging of the stamp from its flat state to a state with a bulge gives rise to an offset which is reproducible only with an accuracy of 100-1000 nm.
WO 2008/087573 discloses the known imprint method in more detail, and also discloses an improvement which involves maintaining contact with a bulged portion of the stamp while the position adjustments are made.
FIG. 1 shows in simplified form the approach disclosed in WO 2008/087573.
A stamp 10 is held by a plate 110. Opposite the plate 110 is a substrate carrier 120 which is movable laterally and thus functions as an actuated chuck. The target substrate (not shown) is mounted on the substrate carrier 120. There is also a reference carrier 122. The reference carrier 122 and substrate carrier 120 are movable with respect to each other to provide the required alignment correction.
A bulge is formed in the stamp 10 and is brought into contact with the reference carrier 122, as shown in the top image. The bulge is increased in size to reach an alignment marker on the substrate carrier 120 so that alignment can be tested, as shown in the middle image.
If alignment is needed, the bulge is reduced in size (in this case bulge width) to release the stamp 16 from the substrate carrier so that the substrate carrier can be moved.
Thus, the top and middle steps shown in FIG. 1 are repeated until the alignment is correct. The process then proceeds with full application of the stamp to the target substrate carried by the substrate carrier 120 as shown in the bottom image.
This process allows alignment measurements to be made with the stamp kept in contact with the reference carrier. The contact ensures that the only effect of the Moiré pattern is caused by an in-plane shift, with no influence of height. The transition from small bulge to full contact is more reproducible than the process of repeating the initial contact after an alignment measurement. As a result, this process enables more accurate control of the stamp alignment.
However, this method requires a special substrate carrier in the form of the substrate carrier and reference carrier (anchor), which is not standard in mask aligner tools for which SCIL add-on tooling is available.
There is therefore a need for an accurate alignment method and apparatus which is suitable for use within an imprint lithography method, and which in that case requires reduced modification to conventional mask aligner tools.